Recently, a variety of communication equipments having a high performance, such as a mobile phone, have been developed rapidly and widely used. Such portable communication equipment requires a high performance power circuit for a power amplifier installed in the portable communication equipment to send a signal.
The power circuit generally includes a DC-DC converter to supply necessary power to the power amplifier. The DC-DC converter may be able to change the output voltage in accordance with an external-control voltage. There are two types of DC-DC converters which are voltage-mode and current-mode DC-DC converters.
A background voltage-mode DC-DC converter generates an output voltage by adding an offset voltage to the output voltage of a power amplifier so that the output voltage of the DC-DC converter is adjusted to a necessary power level of the power amplifier. As a result, efficiency of the power amplifier is improved.
Meanwhile, the current-mode DC-DC converter has a lot of advantages, for example, good liner-regulation, simplicity for phase compensation. Further, it is easy to limit current and increase to have a large capacity by arranging a plurality of the current mode DC-DC converters in parallel. Therefore, the current mode DC-DC converter has been widely used recently.
FIG. 1 illustrates a background DC-DC converter 100. The background DC-DC converter 100 includes a reference voltage generator 112, bleeder resistors RFB1 and RFB2, an error amplifier 110, a PWM comparator 111, a RS-flip-flop 121, a switching transistor M100, an inductor L1, a capacitor C1, a slope voltage generator 115, a driver circuit 116 and a compensation circuit 117.
The reference voltage generator 112 generates a reference voltage Vref. The bleeder resistors RFB1 and RFB2 generate a partial voltage of the output voltage by dividing an output voltage. An operation of the DC-DC converter 100 will be described.
The RS-flip-flop 121 is to be set at a rising edge of a clock signal CLK and an output voltage at an output terminal Q of the RS-flip-flop 121 becomes high level. The output voltage at an output terminal Q is input to a gate of the switching transistor M100 through the driver circuit 116. The switching transistor M100 is turned on.
When the switching transistor M100 is turned on, an inductance current IL of the inductor L1 is increased. The PWM comparator 111 outputs a reset signal to the RS-flip-flop 121 when the inductance current IL becomes a predetermined voltage which is determined by a voltage of the slope voltage generator 115 and a voltage of the compensation circuit 117. The RS-flip-flop 121 outputs a low level voltage at the output terminal Q of the RS-flip-flop 121. Then, the switching transistor M100 is shut off.
After the switching transistor M100 is shut off, a charge stored in the inductor L1 is being fed to the output terminal continuously through the diode D1 as the inductance current IL. The RS-flip-flop 121 is set again at a following rising edge of the clock signal CLK. The above series of the operations are repeated.
The error amplifier 110 amplifies a voltage difference between the reference voltage Vref and the partial voltage of the output voltage. An amplified voltage by the error amplifier 110 is input to non-inverted terminal of the PWM comparator 111 through the compensation circuit 117. The slope voltage generator 115 contributes to avoid sub-harmonic oscillation which may occur when on-duty of the PWM comparator 111 exceeds 50%.
With the background current-mode control DC-DC converter, it is possible to make the power circuit to have a wider dynamic range of the output voltage by some extent. However, there is a large demand for further improvement with respect to a possible voltage range which the power amplifier needs.